The responsibility for security in public areas of U.S. airports is shared by local authorities and TSA, leaving vulnerabilities in the public areas where passengers check in and drop off their bags.

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PARIS — After the attacks in Brussels on Tuesday, many countries quickly added soldiers and police officers at airports and rail and subway stations, hoping to reassure passengers and deter other potential terrorists.

At the same time, the U.S. State Department issued a travel alert warning U.S. citizens to potential risks of travel to Europe, noting that “terrorist groups continue to plan near-term attacks throughout Europe, targeting sporting events, tourist sites, restaurants and transportation.”  The travel alert expires on June 20.

Despite a series of episodes in recent years that have targeted transportation hubs worldwide, security experts predict the latest attacks will revive — but not resolve — a thorny public debate about the benefits of ever more costly and intensive screening systems meant to identify terrorists among the millions of people who travel each day.

“It’s always about achieving a balance between what is achievable and what is practical,” said Norman Shanks, a consultant and former manager of airport security at London Heathrow Airport.

Belgium bombings

German police officers guard  a terminal at the Frankfurt airport. (AP Photo/Michael Probst)
German police officers guard a terminal at the Frankfurt airport. (AP Photo/Michael Probst)

Since the Sept. 11, 2001, attacks in the United States, travelers worldwide have grown accustomed to measures scrutinizing everything they carry with them into an airport, from the soles of their shoes to shampoo bottles, for potential weapons or traces of explosives. But such measures have mainly been focused on preventing terrorists from carrying out an attack aboard an aircraft, rather than an attack on the airport itself.

In the United States, the federal agency responsible for airport screening, the Transportation Security Administration (TSA) — whose budget is $7.3 billion this year — spends much of that money on security screeners at checkpoints and on equipment that can detect bombs or banned items such as knives.

But the responsibility for security in public areas of airports is shared by local authorities and TSA, leaving vulnerabilities in the public areas where passengers check in and drop off their bags.

“We really need to re-examine how we allocate our resources,” said John Cohen, a former acting undersecretary for intelligence and analysis at the Department of Homeland Security and now a professor at the Faith-Based Community Security Program at Rutgers University.

But as the dual explosions at Brussels Airport made clear, fully securing the public areas of a large, sprawling site like an airport is daunting.

The attack Tuesday was not the first time terrorists had targeted the easily accessible areas of a major European airport.

In 2011, a suicide bomber killed 37 people in the arrivals hall of Domodedovo International Airport, Moscow’s busiest. A series of similar attacks occurred in the 1980s, striking airports in Frankfurt, Paris, Rome and Vienna.

In 2013, a shooting at Los Angeles International Airport killed a checkpoint security guard and wounded several other people.

Some countries in recent years have introduced additional layers of security at airports, including mandatory checks of passengers and baggage at building entrances and even of arriving cars, taxis and buses. Such measures are common at many airports in Russia and Israel, for example, and in much of the Middle East and parts of Africa.

Kenneth Kasprisin, a former interim director of TSA, said airports should expand screening of vehicles coming into airports, particularly larger ones.

But analysts said that while such checks could serve as a deterrent to some attackers, they are far from foolproof.

“There are countries where people have to present their passport and ticket at the airport entrance,” Shanks said. “That is designed to keep out nontravelers, but it won’t keep out a bomber, because bombers can still buy tickets.”

Such an approach also assumes the employees charged with performing those checks are appropriately trained to spot a potential threat.

“Sadly, at many airports where I have seen screening on entry, the staff performing this haven’t actually been very efficient or effective,” Shanks said. “If you are a potential bomber, you can monitor how well people are doing their job and exploit those weaknesses.”

Others warned that adding new security choke points outside airport terminals, rather than diminishing the threat, might simply move it to another location.

“If you look at the areas of the world that have experience with suicide bombers, they often tend to blow themselves up at security checkpoints, where there are lots of people standing densely packed together,” said Philip Baum, managing director of Green Light, an aviation-security consulting firm in London. “If we start creating more queues of people to go through more checks, we just create new targets.”

Security experts have long argued that the most effective way to diminish threats to airports and other large public spaces involves a combination of tools, including greater use of surveillance cameras, bomb-sniffing dogs and, in particular, training more security personnel in the use of behavior-monitoring techniques.

“At the moment most airport-security check points are about identifying prohibited items,” Baum said. “So much time is spent taking liquids and aerosols and gels off of people who pose absolutely no threat, rather than differentiating between people and recognizing that different people pose different threats.”

Brian J. Cantwell of The Seattle Times contributed to this report.